Role of lipopolysaccharide and a major outer membrane protein from Francisella tularensis in the induction of immunity against tularemia.

A crude outer membrane preparation from Francisella tularensis Live Vaccine Strain (LVS) was used to immunize mice. Immunized mice were completely protected from a F. tularensis challenge. We evaluated the role of two major outer membrane antigens in the induction of protective immunity, namely lipopolysaccharide and an outer membrane protein FopA. We presented FopA to the immune system using an aromatic amino acid-dependent Salmonella typhimurium as a vector. Although mice mounted an immune response to cloned FopA no significant protection was induced. However, LPS immunized mice were completely protected. We conclude that LPS is a major protective antigen whereas FopA has a limited or no role in the induction of protective immunity.